Long-term gelation of laponite aqueous dispersions

The origin of the long-term gelation of clay suspensions was recently questioned. We have investigated this problem by looking at the chemical stability of the laponite solid particles as a function of the preparation, the long-term storage, and the age of the suspensions. Under ambient atmosphere, Mg2+ is released from the laponite, suggesting that carbon dioxide from the atmosphere promotes acidification of the dispersions, resulting in a progressive laponite dissolution and a slow increase of the ionic strength. These factors induce a sol-gel transition, leading to the observation of fractal aggregates above the micrometric length scale. Such an evolution is not observed if the samples are carefully handled under N-2 atmosphere for a long period of time. In these circumstances, the suspensions stay free of Mg2+,and undergo a fluid-solid transition along a defined transition line in the plane (volume-fraction-ionic-strength). In this situation, one interesting question concerns' the unusual coincidence between a mechanical transition and an incomplete nematic transition.